"An attacker who successfully exploited this vulnerability could run arbitrary code in kernel mode," Microsoft said in a security advisory published Wednesday. "An attacker could then install programs; view, change, or delete data; or create new accounts with full administrative rights."

This is an elevation-of-privilege (EoP) vulnerability, not a remote code execution one, which means that attackers need to already have access to a low-privileged account on the targeted system in order to exploit it.

According to Microsoft, this vulnerability is already being exploited in "limited, targeted attacks," but doesn't affect Windows versions newer than Windows XP and Windows Server 2003.

The company provided a temporary workaround that involves disabling NDProxy.sys, but this will cause certain services which depend on TAPI, like Remote Access Service (RAS), dial-up networking and virtual private networking (VPN), to no longer work.

Microsoft credited security vendor FireEye with helping the company investigate the new vulnerability, which is being tracked as CVE-2013-5065.

This EoP vulnerability is being exploited in attacks in conjunction with a remote code execution vulnerability in older versions of Adobe Reader that was patched in May, FireEye security researchers Xiaobo Chen and Dan Caselden said Wednesday in a blog post. The exploit targets computers running Adobe Reader on Windows XP with Service Pack 3, but users who have the latest versions of Adobe Reader installed should be protected, they said.

According to the FireEye researchers, if the exploit is successful, an executable file is dropped in the Windows temporary directory and is executed.

The exploit is used in targeted attacks, but the FireEye researchers are still investigating the method used to distribute it and the identity of the targets. The exploit installs malware that connects back to and communicates with a few hacked websites, the company said via email. However, other details about what the malware is designed to do have yet to be determined.